Problem: Solve for $t$ : $9 + t = -12$
Solution: Subtract $9$ from both sides: $ \begin{eqnarray} \\ 9 + t &=& -12 \\ \\ {-9} && {-9} \end{eqnarray} $ $ \begin{eqnarray} \hphantom{9 + t} &\hphantom{=}& \hphantom{-12} \\ t &=& -12 {- 9} \\ \end{eqnarray} $ Simplify. $ t = -21$